Ornithine decarboxylase: An indicator for growth of NIH 3T3 fibroblasts and their c-Ha-ras transformants

Growth rates of different clones, all derived from NIH 3T3 mouse fibroblasts, were determined. Four different types of cells were studied: (1) Normal NIH 3T3 fibroblasts; (2) a fast-growing NIH 3T3 clone obtained by repeated passages; (3) transformed clones (obtained by transfecting NIH 3T3 with the oncogene c-Ha-ras); (4) a slow-growing revertant obtained by repeated passages of the transformed line. Growth rates were determined by the following markers of proliferation: thymidine incorporation, protein accumulation and cell number. In parallel experiments growth rates were determined by a new approach based on measuring ornithine decarboxylase (ODC) activity. Transformed cells, which were characterized by phase-contrast microscopy and by electron microscopy grew rapidly and showed high ODC activity. Similarly, a high-passage NIH 3T3 variant, which grew rapidly, also possesed high ODC activity. On the other hand, high-passage of a transformed clone revealed phenotypic changes confirmed by electron microscopy. These cells exhibited reduced growth rates and their ODC activities were similar to those of the normal NIH 3T3 cells. A confident correlation was found among each of the three conventional parameters of growth and between them and ODC activity. However, in all the cases studied ODC activity appeared early in the cell cycle before the expression of the other markers of proliferation. It has been suggested that ODC is a reliable early marker of cell proliferation and might also serve as an important tool for determining the arrest of growth.